Process for the deposition of diamond films using low energy, mass-selected ion beam deposition

ABSTRACT

A low energy (10 to 300 eV), mass-selected ion beam is used to deposit thin films on atomically clean substrate surfaces. For example, a C +  ion beam may be used to deposit a chemically bonded diamond or diamondlike film on a substrate at room temperature. For thin carbon films, the initial monolayer of the deposited film is in the form of a carbide layer which is chemically bonded to the substrate atoms. The film evolves gradually over the next several layers deposited, through intermediate structures, into a diamond structure. The optimum C +  energy range for formation of the diamond structure is about 30 to 175 eV. Below 10 eV the final diamond structure has not been attained and above 180 eV there is a sharp increase in the dose required to attain this final structure. Multiple ion beams may be used to deposit multicomponent films including films doped with very low concentrations of foreign atoms. The diamond films produced by this process are found to be free of impurities, inert to O 2  chemisorption, structurally stable up to 350° C., have a low sputtering yield, and have a sharp interface with the substrate surface. Applications for such films include protective coatings, insulators, and doped semiconductors.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.067,298, filed Jun. 25, 1987 now U.S. Pat. No. 4,822,466.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the production of thin, chemically bondeddiamond or diamondlike films. More particularly, it relates to suchfilms produced by ion beam deposition.

2. Description of Related Art

Deposition of diamondlike carbon films has been the subject of intenseresearch for about thirty years. This research has accelerated markedlyduring the past few years. The basic interest in diamondlike carbonfilms stems from the unique set of physical properties of diamond: it isthe hardest material known; it is an excellent electrical insulator yetis the best thermal conductor known; it has high dielectric strength,and is highly transparent in the ultraviolet, visible and infraredregions of the spectrum; it is chemically inert and therefore resistantto oxidation and corrosion; and, it is biologically compatible with bodytissues.

Attempts to fabricate true diamond films have resulted in carbon filmshaving properties which vary over a range of many orders of magnitude.For example, the electrical resistivity of such films has been reportedto vary between 10⁻² and 10¹² ohm-cm. The unique characteristics of someof these films and the possibility of "tailoring" a combination ofdesired properties for a specific purpose result in many advantages ofsuch films for a variety of applications. Proposed applications include:optical coatings (in particular for hazardous environments and outerspace); protective thin film coatings for magnetic recording media(e.g., computer disks); heat sinks and high thermal conductivitycoatings for semiconductor applications; solid state devices; moisturebarriers; low friction coatings for tribological applications;protective coatings compatible with body tissues for medicalapplications; etc.

The literature on diamondlike films includes several hundredpublications, most of them appearing after 1980.

The study of diamondlike carbon films is complicated by the ambiguousand inconsistent nomenclature which has been used in research on thesefilms. These films are referred to as "diamondlike films", "hardcarbonaceous films", "hard carbon", "a-C:H", and "i-C". In the past,different names have been used to describe materials which are verysimilar while at other times the same name was applied to very differentmaterials. This confusion is also related to the sometimes overlookedfact that the field of carbon films covers several pure phases of carbonas well as hydrocarbon compounds. The two best known crystalline phasesof carbon are graphite, the stable hexagonal form, and diamond, themetastable cubic form. Diamond is stable at very high temperatures andpressures. In the past two decades three additional metastable carbonphases have been discovered: Lonsdalite (also known as hexagonaldiamond); Chaoite (a hexagonal high pressure carbon phase); and, twoother cubic, high pressure phases of carbon. Present data on theproperties of the carbon phases refers to either cubic diamond orgraphite. Very little information is available on the properties of theother phases.

For the purposes of this disclosure, the term "diamond" will be used torefer to a pure carbon material wherein the carbon atoms have sp³hybridization. The term "diamondlike" refers to any carbon deposithaving a mixture of sp² and sp³ hybridized bonds. The fraction of carbonatoms in a particular hybridization state may vary over a wide range.The process of the present invention may be used to deposit diamondlike(as opposed to diamond) films by employing a C⁺ ion beam of very lowkinetic energy (less than about 20 eV). Alternatively, the substratetemperature may be adjusted to favor the formation of a diamondlikefilm. In general, elevated substrate temperatures favor the productionof diamondlike rather than diamond films. The particular temperaturechosen will depend on the substrate to be coated. For example, at asubstrate temperature of 350° C. a low energy, mass-selected C⁺ ion beamwill not form a diamond film on a nickel substrate (see FIG. 11). Incontrast, such a beam will form a diamond film on a gold substratemaintained at 600° C.

The properties of the different phases of carbon appear to be stronglyrelated to the nature of the carbon bond or the electronic structure ofthe carbon. Cubic diamond has an sp³ tetrahedral structure wherein eachcarbon atom is bonded to four different carbon atoms and no "danglingbonds" exist. In contrast, graphite has an sp² structure wherein eachcarbon atom is bonded to only three carbon atoms in a two-dimensionalarrangement where the remaining p-type orbital forms a "dangling bond"(or π electron band). "Amorphous carbon" refers to a carbon matrix thatincludes any possible mixture of sp¹, sp², or sp³ hybridized carbons,and has no crystalline long range order. The term "diamondlike" or"diamond" coating should be reserved for films that possess a true sp³electronic configuration. The term "i-C" refers to films prepared usingions. The term "a-C" refers to amorphous carbon while "a-C:H" refers toa hydrocarbon material wherein the hydrogen content varies between about10% to about 70%. In the latter, the hydrogen-carbon bond can result inan sp³ structure similar to that of diamond with the exception that aC--H bond terminates the three-dimensional diamond lattice, thusweakening the structure. The marked variation in the properties ofdifferent carbon films thus reflects the nature of localizedhybridization, which can range from being graphiticlike to diamondlike.

Graphite is the stable phase of carbon under ambient conditions.Deposition of carbon on various surfaces using thermal carbon speciesthus results in the formation of either graphitic or amorphous carbonfilms. Such films have a high electrical conductivity and a highabsorption coefficient in both the visible and infrared portions of thespectrum. The metastable nature of the diamond phase requires very highpressures and temperatures for formation. Since thispressure/temperature working region is impractical for routine thin filmapplications, two basic approaches have been adopted for diamond filmfabrication:

(a) energetic atomic species (about 10 to 1000 eV) are used for thecreation of localized (about 20 Å) high temperature/high pressureregions called "thermal spikes" in the developing carbon layer. Theenergetic species can be an ionized or neutral atom or acarbon-containing molecule (no additional thermal carbon needed) or anyother ion (e.g., Ar⁺) that is impinging on the evolving filmsimultaneously with thermal carbon species: and,

(b) chemical reactions involving hydrocarbons (e.g., methane) andhydrogen at elevated temperatures that result in the formation ofdiamond layers.

Tailoring the properties of such films to fit specific applications issometimes accomplished by annealing the films during or after depositionusing laser radiation, energetic ions (about 10 to 1000 keV), etc.

The oldest approach for diamondlike film deposition involves the use ofa beam of low energy carbon ions impinging on a substrate surface with aresultant deposition of carbon. Carbon ions and atoms are typicallyproduced by Ar⁺ sputtering of carbon electrodes within a magneticallyconfined plasma operated at a pressure of about 20 to 50 millitorr. Thecarbon atoms may be further ionized in the same plasma environment. Insuch systems, the C⁺ and Ar⁺ ions are introduced into a depositionchamber maintained at about 10⁻⁴ to 10⁻⁶ torr and accelerated towardsthe sample to energies in the range of about 50 to 100 eV.

Ion beam deposition techniques can be divided into severalsubcategories:

(a) primary ion beam deposition techniques wherein carbon/hydrocarbonatoms are generated, extracted, provided with a controlled amount ofenergy, and directed onto the substrate. The carbon/hydrocarbon ions canbe mass-selected. The carbon ions in these systems are used for both thecarbon supply needed for film formation and the energy source for the"thermal spikes" needed for diamond structure formation;

(b) ion beam sputtering deposition techniques wherein an energetic ionbeam (usually inert gas ion beam) is directed onto a graphite target andthe resulting sputtered carbon atoms and ions deposited on thesubstrate. The energy distribution of these carbon species depends onthe nature of the primary ion beam, the ion energy, and the angle ofincidence; and,

(c) dual ion-beam techniques wherein, in addition to the carbon flux ofeither (a) or (b) above, a second inert reactive ion beam simultaneouslyimpinges on the substrate to be coated. The carbon flux can consist ofeither energetic or thermal species since "thermal spikes" are generatedby the additional ion beam. The complementary ion beam increases the"diamondlike" constituent of the films by preferential sputtering of thegraphitic/amorphous carbon regions.

Different ion beam deposition systems differ markedly in intrinsicposition parameters such as the nature and energy distribution of carbonspecies, beam flux density, ambient pressure during deposition, andcomposition of non-carbon species in the impinging flux. Thesignificance of some of these deposition parameters for diamondlike filmformation is discussed below.

Various plasma deposition techniques have also been used to producediamondlike films. Plasma decomposition of various hydrocarbon gasesresults in the deposition of carbon films on substrates placed on anegatively biased electrode. Radio frequency, DC and pulsed plasmasystems have been used. Several process parameters related to thistechnique can be varied and controlled, such as the type of hydrocarbongas, plasma decomposition power, and substrate bias. Often, anargon/hydrocarbon gas mixture is used, resulting in Ar⁺ bombardment ofthe evolving film that preferentially sputters and partially removes thegraphitic and amorphous carbon constituents of the film. A relativelyhigh hydrogen content is typical for these techniques.

Chemical vapor deposition (CVD) processes have also been used to producediamondlike carbon films. The basic principle of CVD diamond filmformation is the use of chemically active hydrocarbon fragments (ionsand radicals) for the spontaneous growth of diamondlike material underrather metastable conditions. In a typical experiment, a mixture ofmethane and hydrogen gas (about 1% methane) is introduced into thesystem and hydrocarbon fragments, atomic hydrogen, and carbon speciesare generated using an excitation source (hot filament, radio frequencyor microwave plasma). Diamond films are deposited on substratesmaintained at a temperature in the range of about 100°-1000° C. Suchdiamond film formation is strongly dependent on the methaneconcentration and the substrate temperature. The use of methane is verycommon, but other hydrocarbons have been successfully used, sometimesresulting in a higher deposition rate. The true crystalline diamondstructure of CVD films is now well established.

Discussed below are the parameters related to carbon deposition. Thisdiscussion is independent of specific deposition techniques and ispresented in terms of the influence of the parameters on the growth andfinal form of the carbon film.

Nature of Impinging Parent Species

A large variety of impinging parent species can be used, includingcarbon ions (both positive and negative) and carbon atoms, differenthydrocarbon radicals and ions, and carbon clusters along with non-carbonspecies such as hydrogen and argon. The nature of the parent speciesinfluences the flux of the impinging carbon and the stickingprobability, thereby affecting the deposition rate. The nature of thespecies used may also affect the properties and structures of theas-deposited carbon films. Films formed by hydrocarbon discharge systemstypically have a large hydrogen content (H/C atomic ratio can vary fromseveral percent to 100 percent). The exact composition of the flux whichimpinges on the substrate, even within a very narrow concentrationrange, can be crucial for the formation of diamondlike films withspecific properties. Some CVD processes, for example, usemethane/hydrogen mixtures where the methane concentration is in therange of about 0.3 to 2%. A methane concentration change even in thislimited range has been found to strongly influence the film evolution.

The mechanisms involved in the formation of diamondlike films are notwell understood. Three different species that may play an important rolein these processes (in addition so carbon) are thought to be hydrogen,hydrocarbons, and argon. Hydrogen is believed to be important for twodistinct purposes: (a) stabilization of carbon sp³ bonds and saturationof "dangling bonds" (by atomic hydrogen); and, (b) selective etching ofgraphitic and amorphous carbon domains by the formation of hydrocarbonsthat evolve from the films. Hydrocarbons appear to be essential for theformation of diamond in CVD processes where the diamond metastable phaseis formed by chemical reactions associated with hydrocarbon radicals.Argon ions are believed to etch the non-diamond constituents of thegrowing films due to their high sputtering efficiency for graphite andamorphous carbon compared to diamond. In most practical systems,measurement and control of the composition of the parent incidencespecies are difficult. This results in an irreproducibility of theprocess and a consequent spread in the properties of the as-depositedfilms.

Ion Energy

Ion energy is a very important parameter for diamondlike film depositionin most techniques, with the possible exception of CVD processes. Ionenergy may affect the film evolution in several ways:

(a) In the initial stage it can force the formation of metastable phases(e.g., of carbidic nature) that can contribute to better adhesionbetween the film and the substrate.

(b) The ion energy is responsible for the "thermal spikes" in the filmthat are essential for the formation of metastable carbon phases. Aminimal ion energy on the order of several eV may be necessary.

(c) ion energies of several tens of eV and above (depending upon theparent species involved and the angle of incidence) are necessary forpreferential sputtering of graphic domains and for increasing thepercentage of sp³ hybridized carbon in the final product. Theself-sputtering of the film by energetic carbon ions may be a limitingfactor in achieving high deposition rates, for at high ion energies thesputtering rate can exceed the deposition rate. Another beneficialeffect of the preferential sputtering is the possible removal of surfaceimpurities.

(d) Ions with energies of several hundreds of eV and above (dependingupon the parent species involved) can damage the evolving structure bycreating atomic displacements, thereby destroying the sp³ nature of thedeposit. Elevated deposition temperatures (approximately 400°-700° C.)can, however, anneal the damage resulting with diamond formation.

(e) Ions with energies of one keV and above are implanted into the film.Under appropriate conditions that allow annealing of defects, internalgrowth of diamond occurs. In many practical systems, the energydistribution of the species used for deposition is very broad anduncontrolled, resulting in irreproducibility of the final product.

Another important factor is the concentration depth profile and damagedepth profile of the impinging ions or neutral atoms, especially at thehigher energies. These profiles are usually nonuniform with a Gaussianshape having a maximum with a depth determined by the nature of theparent ions, the nature of the substrate, and the ion energy. Filmshaving properties which vary as a function of depth can thus be formed.

Incident Carbon Flux

The effect of the incident carbon flux on the final properties of thefilm is not well understood, although it has been established that theflux is one of the dominant parameters that control the structure andproperties of films in general. For example, the power of radiofrequency plasma sources has been found to have a strong effect on theoptical properties of carbon films (e.g., absorption coefficient orenergy bandgap) which could be attributed to a flux effect. It has alsobeen reported that the flux markedly changes the temperature range wherediamond is formed during C⁺ ion implantation. Apart from the influenceof the flux on the growth mechanism and the final microstructure of thefilms (a phenomenon generally observed in thin film technology), it canalso be associated with secondary effects that may influence the finalproperties of the carbon films, such as: (a) increasing substratetemperature; (b) increasing ambient pressure; (c) increasing chargingeffects when ions impinge on an insulating substrate; (d) producingmultiple collision effects in energetic particle bombardment; and, (e)increasing the ratio between the impinging carbon flux and the ambientgas pressure, thus reducing the concentration of trapped impurities inthe carbon film.

Nature of the Substrate

The properties of the substrate strongly influence the carbon filmevolution, a well-known phenomenon in thin film technology. Thesubstrate material may affect the carbon film in different ways.Materials forming stable carbides are more likely to form stronglyadhering films and usually have a high sticking probability for carbonparticles, the carbide serving as an intermediate layer between thesubstrate and the final film. The carbon/carbide solubility in thesubstrate should also be considered since energetic carbon species candiffuse into the substrate instead of forming a stable carbon surfacelayer. The match between substrate lattice and diamond lattice isimportant if epitaxial growth is considered. Diamond films have beensuccessfully grown on diamond seeds, using several depositiontechniques. Nickel (111) has a lattice constant similar to that ofdiamond and has been used for achieving epitaxy. The crystalline sizeand orientation of the substrate is another parameter that determinesepitaxial growth. Correlations between the diamond lattice and thesubstrate orientation have been found, for example, in the case of CVDdeposition on Si(100) substrates. Surface roughness can also affect thecrystalline growth of diamondlike films, scratches serving as nucleationsites for the formation of crystals. Diamondlike films have beensuccessfully prepared on different materials with different substratefeatures, including smooth and rough, single/polycrystalline andamorphous. An additional substrate factor that should be considered isthe cleanliness of the substrate surface. In many systems, sputteringand annealing of the substrate surface can be performed in order toachieve a clean surface prior to carbon deposition.

Substrate Temperature

Successful ion beam and plasma depositions of diamondlike films havebeen carried out at room temperature and lower temperatures (liquidnitrogen). In contrast, CVD processes typically need a reactiontemperature of about 800°-1000° C. Attempts to produce such films attemperatures higher than about 100° C. have, in some cases, failed. Itappears that in these cases the sticking coefficient of carbon specieson surfaces having an elevated temperature was small. Thetemperature-dependent condensation rate effect for nucleation of plasmapolymerized hydrocarbon films has been given as a possible explanationfor this observation. Another effect that can prevent diamondlike filmformation at elevated temperatures is the diffusion of carbon into thesubstrate. Such an effect has been observed for carbon films depositedon Ni(111) and subsequently annealed to temperatures higher than 400° C.If the deposition rate is lower than the diffusion rate, film formationdoes not occur. The formation of gaseous carbon compounds at elevatedsubstrate temperatures is plausible for hydrocarbon deposition or dualcarbon/hydrogen deposition, but is unlikely for ion beam depositionwhere no hydrocarbons or hydrogen is present. The formation at elevatedtemperatures of graphitic deposits that are removed by energeticparticle sputtering is a more reasonable explanation for thelast-mentioned system.

Deposition on substrates heated to about 300° C. and above haveresulted, in some cases, in the formation of graphiticlike films asrevealed by low electrical resistivities (about 100 to 0.1 ohm-cm) and adecrease in hardness compared to films deposited on similar substratesat lower temperatures. For either low energy C⁺ deposition on diamond orhigh energy C⁺ implantation into diamond followed by internal diamondgrowth, temperatures in the range of about 400°-700° C. were needed fordiamond formation.

As mentioned previously, CVD diamond films, in contrast to ion beam andplasma deposited films, require a substrate temperature of about800°-1000° C. This may result in formation of a thick carbide layerbetween the substrate and the diamond film due to the high diffusionrates at such elevated temperatures. Such conditions may thereforeeliminate many possible applications where thick interface layers orhigh temperatures are intolerable.

Ambient Pressure

The typical pressure in most deposition systems for diamond ordiamondlike films is 10⁻⁶ torr or higher. Comparatively high partialpressures of gases like hydrogen, argon, and hydrocarbons are sometimesan essential element for the deposition scheme. Some researchers are ofthe opinion that diamondlike film formation is not dependent on the gaspressure; however, this remains to be proven. In many casescomparatively high oxygen and nitrogen concentrations are found in thefilms, which may affect some of their properties. Uncontrolled vacuumconditions may be one of the reasons for the poor reproducibility ofmany diamondlike deposition systems.

The role of the partial pressure of molecular hydrogen in thestabilization of diamond sp³ bonds in diamondlike films is alsoquestionable. It is well-known that atomic hydrogen does play a role inthe stabilization of real diamond surfaces. However, very hightemperatures (about 100° C. and above) or an auxiliary power source isneeded for atomic hydrogen formation, in addition to sufficient H₂pressure. Another beneficial effect of atomic hydrogen is the selectiveetching of graphitic domains resulting in a high percentage of diamond.Hydrogen is several orders of magnitude more efficient in graphiteremoval than in diamond removal.

The technique of chemical vapor deposition (CVD) has perhaps been themost widely attempted method for the deposition of diamond films onsubstrates. In chemical vapor deposition, a hydrocarbon is pyrolyzed byheat or radiation to an ionized gas/electron mixture which is allowed todeposit on an exposed substrate. To inhibit the formation of thegraphite form of carbon in the deposited film, hydrogen or a hydrogenplasma is typically added to the hydrocarbon before it is decomposed.The resulting diamond films are typically diamondlike, rather than truediamond, and contain an undesirable content of hydrogen.

In ion beam deposition techniques, a substrate is bombarded by highvelocity ions. Again, films reported to have been deposited by suchtechniques are diamondlike rather than true diamond films.

Experiments with low energy reactive ion bombardment of surfaces usingions of carbon, nitrogen, and oxygen have shown that both the gaseousreaction products and the surface film growth and properties are verysensitive to the energy and momentum of the impinging beam species.Solid state phases in the surface region that are far from thermodynamicequilibrium, such as unusual metastable structures, can be formed due tothe condition of high available activation energy which is rapidlyquenched by the solid. An example of this phenomenon is the productionof insulating carbon films by a variety of plasma and ion beamtechniques. These carbon-based films have been shown to be mechanicallyhard, chemically resistant, and optically transparent, while having aresistivity, refractive index, lattice constant, dielectric constant,optical absorption edge, and valence-band structure similar to that ofdiamond. Hence, the appellation "diamondlike" films has developed, eventhough the properties of these films can vary considerably depending onthe method used for their production. The growth mechanism and optimumconditions for obtaining "diamondlike" properties for such films are ofobvious interest.

ion beam deposited carbon films are in metastable amorphous or quasiamorphous states whose relative stability and physical properties may bestrongly dependent upon incorporation of constituents like hydrogen,oxygen, etc. within the structure.

The techniques of the prior art have failed to provide a stronglyadhered (preferably chemically bonded), pure carbon film on a substratearticle which has a true diamond composition or which closelyapproximates the microstructure and physical properties of true diamond.

The ability to reproducibly form thin carbon films with a diamondstructure has important technological applications, not only because ofthe strength and hardness of diamond, but because it has an extremelylow electrical conductivity (i.e., it is an excellent electricalinsulator) and has the highest thermal conductivity of all knownsubstances (i.e., it is an excellent heat conductor). These propertiesstem from the strong, rigid, symmetrical tetrahedral bonding betweencarbon atoms. This bonding produces an extremely elastic lattice withlow phonon amplitudes, that is, the thermal motions have limitedamplitude; as a result, the diamond allotrope of carbon has the lowestspecific heat of all elements and high heat transfer rates.

SUMMARY OF THE INVENTION

In the process of the present invention, low energy (about 10 to 300eV), mass-selected, C⁺ ion beams are used to deposit thin carbon filmson selected surfaces in an environment maintained at less than 10⁻⁸ torrat room temperature. The resulting films may be characterized by Augerelectron spectroscopy (AES), x-ray and UV photoelectron spectroscopy(XPS and UPS), valence level electron energy loss spectroscopy (ELS),K-shell ionization loss spectroscopy (ILS), and ellipsometry.

It has been found that for surfaces of silicon (100), nickel (111),tantalum, tungsten, and gold, the initial monolayer of the depositedfilm is in the form of a carbide layer which is chemically bonded to thesubstrate atoms. The film evolves gradually over the next several layersdeposited, through intermediate structures, into a diamondlikestructure. The diamondlike structure of the outer layers of such a filmmay be confirmed by comparing the results of the above-mentionedspectroscopic measurements with those of pure diamond and graphite andby performing band structure calculations.

The films produced by the process are generally free of impurities,inert to O₂ chemisorption, structurally stable up to 350° C., have a lowsputtering yield, and have a sharp interface with the substrate surface.

It has been found that films having an Auger spectrum corresponding to adiamondlike structure are obtained only for certain combinations of ionenergy and dose and that outside of the range, other carbon formsprevail.

The process is practiced with a low-energy, mass-selected, active-ionbeam in an apparatus which provides for efficient differential pumpingin order to maintain a clean target surface. In situ techniques foranalysis of the reaction products allow the growth mechanism to beobserved and optimum conditions for obtaining diamondlike properties forsuch films to be determined.

In one embodiment, the process utilizes an apparatus wherein C⁺ ions aregenerated by electron impact of CO gas and are extracted from the ionsource region at the potential of interest, accelerated to high energy,mass-selected to pass ¹² C⁺, and transported through a long flight tubedesigned to eliminate fast neutrals. The ions are decelerated just priorto impact with the substrate (target). It is easier to transport andfocus a high-energy beam. When decelerated, ion beams tend to diverge.Hence, the deceleration is most preferably positioned as close aspossible to the substrate (e.g., on the order of a few millimeters). Theefficient differential pumping of the beam line allows maintenance ofthe system pressure in below 10⁻⁸ torr during exposure of the substrateto the C⁺ ions. In a typical experiment using such an apparatus, the C⁺beam currents are in the range of 10 to 1000 nA and a produce spot sizeof 0.12 cm² with an energy spread of 1 eV. The sample target (substrate)may be, for example, a single crystal of nickel with a polished (111)surface cleaned by 3-keV Ar⁺ sputtering and electron-beam annealing to900° C. before each experiment.

Carbon films with a diamond or diamondlike structure that are chemicallybonded to surfaces can be deposited by means of low-energy C_(m) ⁺ ionbeams according to the process of the invention. When mass-selectedC_(n) ⁺ beams at energies in the range of from about 20 to 200 electronvolts impinge on an atomically clean surface, the first carbon monolayergrows as a carbide structure that is chemically bonded to the surface.As deposition continues, the structure evolves over the next severalatomic layers into a diamondlike structure. These pure carbon films arestrongly adhered to the surface through the carbide bonds, which alsoprovide for an intimate interface. The nature of the carbon deposit andthe evolution of the film can be intimately followed by means of thecarbon Auger lineshape. These lineshapes serve as a fingerprint of thechemical environment.

The deposition apparatus may include multiple ion beam lines for thesimultaneous deposition of different constituents. The beams may befocused to one spot on the substrate for simultaneous deposition offilms composed of different constituents or for doping a film with verylow concentrations of foreign atoms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view schematic drawing of an ion beam depositionapparatus which has been used to develop and practice the process of thepresent invention.

FIG. 2 is an unfolded side view schematic drawing of the ion beamdeposition apparatus shown in FIG. 1.

FIG. 3 is a top view schematic drawing of a multibeam apparatus suitablefor the practice of the process of the present invention. The apparatusillustrated comprises dual ion beams and a deposition chamber equippedwith provisions for in situ surface analysis techniques. An apparatus ofthis type permits the simultaneous deposition of two different specieson a substrate article.

FIG. 4 is a phase diagram as a function of C⁺ dose and energy for carbondeposition on Ni(111). The characteristic c Auger lineshapes have beenused to map out this phase diagram for C⁺ deposition on Ni(111) as afunction of C⁺ ion dose and energy as shown. Regions A, B, and Ccorrespond to films that show the AES lineshapes depicted in FIG. 5 asspectra a, b, and c, respectively. The final AES lineshape (FIG. 5d)evolves smoothly from that of FIG. 5c, and it is not possible to assignan exact dose to the crossover point. Hence a phase corresponding toFIG. 5d is not shown in this figure (FIG. 4). The hatched regionsrepresent transition zones from one form to another.

FIG. 5 shows the evolution of the AES lineshape from (a) carbidic to (d)diamond for 75 eV C⁺ deposition on Ni. The ion doses are: (a) 2.0×10¹⁵ ;(b) 6×10¹⁵ ; (c) 9×10¹⁵ ; and, (d) >×10¹⁶ ions/cm². Spectra (a)-(c) werereferenced according to carbide and graphite spectra of Coad et al.,Surf. Sci. 25, 609 (1971), while spectrum (d) was referenced toaccording to the diamond spectrum of Pate, Surf. Sci. 165, 83 (1986).The inset shows the C KLL signature of a deposited carbon film onNi(111) obtained with minimal exposure to the electron beam. This isprecisely the KLL signature of single crystal diamond.

FIG. 6 shows x-ray photoelectron spectra (XPS) of the C 1s line andcharacteristic energy loss features for graphite and a deposited diamondfilm on Ni(111).

FIG. 7 depicts He II (40.8 eV) ultraviolet photoelectron spectra (UPS)of (a) clean Ni(111) and the same surface after deposition of (b) 4×10¹⁵; (c) 9×10¹⁵ ; and, (d) 3×10¹⁶ C⁺ ions/cm². The spectrum of graphite isshown as (e).

FIGS. 8(a)-8(b) present valence level electron energy loss spectra (ELS)of graphite and a diamond film deposited on Ni(111) measured withelectron beam energies of 150 and 350 eV respectively. Energies of thestructures P_(i) are listed in Table I of Kasi et al., J. Chem. Phys.88, 5914 (1988).

FIG. 9 displays K-shell ionization loss spectra (ILS) for graphite and adeposited diamond film on Si(100). The energies of the K_(i) featuresare listed in Table I of Kasi et al., J. Chem. Phys. 88, 5914 (1988).U_(i) represents the half-height of an ILS peak.

FIG. 10 presents Auger line intensity of the indicated metallicsubstrates as a function of C⁺ ion dose at 150 eV. The differences inthe graphs indicate different growth rates on different substrates. Thetwo stages of behavior for Si and Ni is indicative of a different growthmechanism than that obtained on Au.

FIG. 11 shows carbon and nickel Auger line intensities of annealeddiamond film on Ni(111) at different temperatures. Stage I (T<200° C.)features a very low intensity nickel line and a high intensity, steadycarbon line. Stage II (200° C.≦T<400° C.) features an increase in thenickel line intensity at about 200° C. to a steady level with no changein the carbon line intensity or lineshape. Stage III (T≧400° C.) ischaracterized by a sharp increase in the nickel line intensity; a sharpdecrease in the carbon line intensity; and, transformation of the carbonlineshape from diamond shape to graphitic shape.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENT

An apparatus suitable for practice of the process of the presentinvention includes a source of low energy (about 100 to 300 eV) carbonions with a beam line for mass and energy selection and elimination offast neutrals and a deposition chamber which can be maintained underpressure conditions less than 10⁻⁸ torr during deposition. The apparatusused in the discovery of this invention included a deposition chamberwhich also functions as an analysis chamber for in situ spectroscopicinvestigation of the deposited films. Although convenient, provision forsuch analytical tools would not be necessary in a production apparatus.

The method of the present invention is a deposition process whichfeatures accurate control of the energy of the impinging ions. Variationof the beam velocity is an attribute and works as follows. Relativelylow energy (about 1 to 75 eV) ions can be used to deposit the initialthree to four layers. The gas phase species have the highest sticking orreacting coefficients at such energies. Additional overlayers may thenbe deposited at energies greater than about 75 eV where higher beamcurrents can be achieved and the higher energies permit good atomicmixing in the layers. Such a process can allow for self-annealing ofintrinsic growth defects.

In one particular apparatus used to practice the present invention, theions are accelerated, mass analyzed, and energy selected, while theytravel through four stages of differential pumping and a neutral stop.Deceleration occurs immediately before the ions impinge on the targetsample. C⁺ beam currents used were in the range of 10 to 100 nA in aspot size of 0.12 cm² with an energy spread of 2 to 3 eV; these currentswere stable to within 10% ever a period of 3 to 4 hours. The depositionchamber consisted of a 14-inch bell jar with the sample to be coatedmounted in the center of the bell jar on a movable sample holder. Thesystem was equipped with a Perkin-Elmer double pass cylindrical mirroranalyzer for AES, UPS, and XPS and hemispherical grid LEED optics. Priorto carbon deposition the substrate was cleaned by repeated cycles ofinert gas sputtering and annealing until AES measurements revealed lessthan one percent of contaminant CO. Deposition of carbon was interruptedperiodically, by closing a gate valve in the beam line, in order tocarry out spectroscopic measurements at intermediate ion doses. The doseand energy calibration procedures used are described by Kang et al. inJ. Chem. Phys. 88, 5882 (1988), the teachings of which are incorporatedherein by reference.

The substrate surface was cleaned in situ by Ar⁺ bombardment (10 μA/cm²)and by annealing to high temperatures (approximately 800° C.) prior todeposition. The apparatus employed includes provisions for in situsurface spectroscopic techniques such as x-ray and ultravioletphotoelectron spectroscopy (XPS and UPS), Auger electron spectroscopy(AES), and electron energy loss spectroscopy (ELS), as well asspectroscopic ellipsometry for characterization of the deposit.

An apparatus suitable for the practice of the process is shownschematically in FIGS. 1 and 2. The illustrated apparatus comprises anion accelerator attached to a surface analysis chamber. Importantfeatures of the system are a relatively low-energy ion beam that ismass-selected and has a narrow energy spread; efficient differentialpumping to keep the source gas out of the collision chamber; and, anultrahigh vacuum (UHV) collision chamber with means for positioning andcleaning the substrate article to be coated (e.g., by sputtering andannealing). Also illustrated as a part of the apparatus are the devicesfor the in situ analytical techniques used for characterizing thesurface produced by operation of the apparatus.

FIG. 2 is a schematic drawing in side view of the ion beam depositionapparatus shown in FIG. 1. The components comprising the depositionapparatus of FIGS. 1 and 2 are: an ion source and gas inlet 1;electrostatic quadrupole doublets 2; vacuum pumping port 3; ceramicisolator for flight tube 4; a 60-degree sector electromagnet 5; a gatevalve 6; 6-degree deflector plates (for the elimination of fastneutrals); and turbomolecular pumping stage 7; rotatable flap serving asa differential pumping baffle and beam aperture 8; accelerator lens 9;LEED/AES hemispherical grid analyzer 10; HeI/HelI UPS source 11; CMAanalyzer 12; view port 13; and residual gas analyzer (RGA) massspectrometer 14.

For time-of-flight (TOF) scattering and recoil measurements in thisapparatus, LEED/AES analyzer 10 may be replaced by a massspectrometer/TOF drift tube and RGA mass spectrometer 14 may be replacedby a pulsed mass-selected rare gas ion scattering source. The x-raysource for XPS is located above the UV source out of the plane of theview illustrated. An electron gun for AES is on the CMA axis.

The surface analysis chamber of the illustrated apparatus is a 14-inchstainless steel bell jar. It contains a cylindrical mirror analyzer(CMA) with x-ray and UV sources for photoelectron spectroscopy (XPS andUPS) and an electron gun for Auger electron spectroscopy (AES), a180-degree hemispherical retarding grid for low energy electrondiffraction (LED), and a quadrupole mass spectrometer. In order toperform time-of-flight (TOF) ion scattering and direct recoiling, themass spectrometer/TOE drift tube may be placed in the position of theLEED optics and a pulsed mass-selected rare gas ion source placed in theposition of the RGA mass spectrometer. This provides a scattering andrecoiling angle of 30 degrees and a flight path of 32.5 cm.

The deposition chamber in the illustrated apparatus is pumped by meansof a 500 liter per second turbomolecular pump, a 250 liter per secondion pump, and a titanium sublimation pump with a liquid nitrogen cooledcryobaffle. The resulting base pressure in the deposition chamber of theillustrated apparatus is 3×10⁻¹¹ torr. It is preferable that thedeposition chamber be maintained at a pressure of about 10⁻⁸ torr orless during deposition of a carbon film on an atomically clean substratesurface. If a pressure greater than about 10⁻⁸ torr is permitted todevelop in the deposition chamber, the substrate surface will likely becontaminated with a monolayer or multiple layers of oxygen, hydrogen,water vapor, carbon monoxide, or other residual gas components whichinterfere with, and perhaps even altogether preclude, the formation of abase carbide-bonded carbon layer on the substrate surface by 12 C⁺ ionbeam bombardment. Moreover, if pressures greater than about 10⁻⁸ torrdevelop in the deposition chamber during deposition, the residual gasconstituents will likely become incorporated or entrapped within thedeveloping carbon film and will adversely affect the ability to evolvethe film into one of a diamond structure.

For the purposes of this disclosure, "atomically clean" as applied tosubstrate surfaces means that the surface to be coated is substantiallyfree of foreign (i.e., non-substrate) species, particularly carbon andoxygen. If a surface is atomically clean, sensitive surface analyticaltechniques such as AES will not detect the presence of contaminants suchas adsorbed carbon and oxygen. This implies that there exists less thanabout 0.01 atomic layer (or less than about 10¹³ atoms/cm²) of suchcontaminants on the surface. Many substrates may be treated bysputtering and annealing to provide an atomically clean surface.However, maintenance of such a surface in an atomically clean staterequires UHV conditions (approximately 10⁻⁸ torr or better). Duringdeposition of a film at relatively high ion currents, a somewhat higherpressure may suffice (say, 10⁻⁷ torr) if the arrival date of theimpinging ion species is substantially greater than the arrival rate ofbackground species on the surface being coated. In this regard, itshould be noted that in an experiment wherein the pressure in thedeposition chamber was about 10⁻ 6 torr, a C⁺ ion current of about 1 mAdid not exclude oxygen from films deposited on silicon, nickel, andgermanium substrates. The oxygen in this films was observed by AES.

The beam line of the apparatus illustrated comprises an ion source, lenstransport system, mass analyzer, and decelerator. Construction is ofstainless steel with copper-sealed flanges except in the ion source andmass analysis regions where Viton o-rings are used. Four stages (FIG. 2)of differential pumping are used in this apparatus in order to reducethe pressure from about 1×10⁻⁴ torr in the ion source region to aworking pressure of about 2×10⁻¹⁰ torr or below in the depositionchamber.

The source and mass analysis regions are pumped by two separate 325liter per second oil-trapped diffusion pumps using polyphenylether oil.The entrance and exit apertures of the mass analyzers (each 2 mm wide by1.5 cm high) serve as differential pumping baffles. The region of the6-degree bend (FIG. 2) is pumped by means of a 500 liter per secondturbomolecular pump and a 3 mm diameter circular aperture which admitsthe beam into the main chamber.

The low-energy ions are produced by a Colutron ion source with theionization region maintained at approximately 1 to 300 eV above thesample. The sample is typically grounded, but may, in fact, be biased toany potential so long as appropriate potential differences in the systemare maintained. The Colutron ion source is a plasma-type source whichproduces small size (about 2 mm diameter), intense beams having a narrowenergy spread (about 0.1 to 0.2 eV). The ions are extracted from theionization region by means of a drawout plate. They are then acceleratedto about 1500 eV (the fixed energy chosen for mass analysis) by means ofa two-tube accelerating lens. In the apparatus illustrated, all the beamcomponents downstream from this lens (with the exception of thedecelerator) are maintained at a potential of -1.5 kV relative to thetarget. An electrostatic quadrupole doublet receives the beam from theaccelerator lens. The quadrupole doublet projects the beam through theentrance slit of the magnetic analyzer and an identical quadrupoledoublet receives the beam after it emerges from the exit slit of themass analyzer. This component functions as a stigmatic lens which iscapable of having different magnifications in two perpendicular planesso that the beam shape can be converted from circular to rectangular(for mass analysis) and then back to circular.

In the apparatus illustrated the mass analyzer is a 60-degree sectorelectromagnetic for which the M/E species is selected by varying thecurrent through the magnet. The vacuum housing of the mass analyzer iselectrically isolated from the rest of the vacuum housing and pumps sothat it can be maintained at a potential of about -1.5 kV relative tothe ion source.

Mass selection of the ion beam may be accomplished by any suitabletechnique. Magnetic sector analyzers are preferred over quadrupoleanalyzers since they typically have better focusing and the absence of astraight-through path in such analyzers helps eliminate fast neutralsfrom the beam. The addition of a deflector plate near the exit of theion beam apparatus to further exclude fast neutrals is preferredinasmuch as some fast neutrals do pass through the magnetic sector as aresult of the relatively higher pressures in that portion of the beamline.

The ion beam emerging from the exit of the mass analyzer is reshaped bythe exit quadrupole double lens and then drifts at about 1.5 keV througha parallel plate condenser which bends the beam by about 6 degrees inorder to eliminate the line-of-sight neutral beam which also emanatesfrom the mass analyzer. The beam passes the final aperture into thedeceleration lens which is located in the deposition chamber close toand in front of the substrate. In this way the ions are deceleratedclose to the substrate, thereby maximizing the ion current andminimizing space charge dilation. Suitable decelerators are described inEnge, Rev. Sci. Instrum. 30, 248 (1959), M. L. Vestal, Ph.D.dissertation University of Utah, 1975, p. 37, and Vestal et al., Rev.Sci. Instrum. 47, 15 (1976). The deceleration lens may be operated ineither the step-potential or exponential potential gradient retardationmode. The former mode is employed by maintaining all 34 deceleratorplates at about 1.5 kV while placing 3 variable plates at the exit endof the lens near ground potential. A Faraday cup (1.32 mm diameter)mounted on the substrate manipulator may be used far monitoring the ionflux,

The total beam path length in the apparatus illustrated is 2.4 metersfrom the source to target, providing long drift times for relaxation ofexcited species formed in the source. For low-mass ions, this time forrelaxation is the sum of the flight time through the beam line(approximately 25 microseconds) and the residence time in the source(estimated to be about 10 to 30 microseconds).

Ion current densities of up to 10 microamps per square centimeter wereachieved for a 150 nanoamp beam of which 135 nanoamps were focused intothe Faraday cup (0.0137 cm²). This corresponds to approximately 6×10¹³ions per square centimeter per second striking the surface. Moretypically, 60 nanoamp currents in the Faraday cup were used, giving 3.6microamps per square centimeter and 2.3×10¹³ ions per square centimeterper second. Below 7 eV, the current decreases rapidly, while above thisvalue it increases monotonically. This phenomenon suggests thatspace-charge expansion below 7 eV is the limiting effect on the finalion current. Transmission through the beam line is independent of thefinal energy since all voltages are referenced to the potential of theionization region. When this potential changes, every other voltagechanges with it. Thus, when the potential of the ionization region isvaried with respect to a grounded substrate (to vary the final beamenergy), the velocities, and hence transmission characteristics of theions in the beam, do not change. All changes with varying beam energyoccur in the step potential decelerator.

Approximate current density profiles of the beam were obtained by twomethods: (1) scanning the Faraday cup aperture through the beam; and,(2) depositing a reactive species on a surface and using AES to monitorthe reacted spot by scanning the surface using the AES electron beam.The beam current is stable over a period of hours to within two percent.Experiments with apertures show that the beam angular divergence is lessthan one percent before the deceleration region and increasessubstantially beyond this region up to values less than about sixpercent.

C⁺ ion beams can be produced by a variety of ion sources, such aselectron impact, plasma, sputtering, and discharge sources. In theapparatus illustrated, the C⁺ ions are produced by admitting acarbon-containing gas into the discharge region. Preferred ion sourcesare those which provide a relatively high current with a relativelynarrow energy spread. A narrow energy spread is desirable inasmuch as itallows one to use relatively large slits in the magnetic sector to passa high ion fluence through the mass analyzer. Sputter sources typicallyproduce an ion current with a very broad energy spread. More preferredare duoplasmatron or Freeman-type sources wherein an electron beam isused to ionize a gas. Such sources provide ions with a tight energyspread, typically about ±1 eV. For the deposition of diamond ordiamondlike films beams of C_(n) ⁺ or C_(n) ⁻ where n is an integergreater than or equal to one are preferred. Suitable gases for theproduction of such beams include, but are not limited to carbon monoxideand hydrocarbons, as is well-known in the art. Carbon-containing gasessuitable for production of C⁺ ions include carbon monoxide, carbondioxide, and hydrocarbons. Carbon monoxide was used in the apparatusillustrated. A typical composition of the ensuing ion flux for a carbonmonoxide gas is CO⁺ (78%), C⁺ (18%), and O⁺ (4%). The individual ionsare mass-selected by varying the current through the magnetic sector andare focused on the sample target by tuning the electrostatic lens. Thesubstrate particle target is atomically cleaned, preferably by rare gasion sputtering and electron beam annealing to temperatures just belowthe substrate's melting point. In the illustrated apparatus, for anickel substrate, 3 keV Ar⁺ ions were used and annealing was performedat 900° C. LEED may be used to check the surface structure followingthis cleaning procedure. Preferably, the deposition chamber ismaintained at a pressure of 1×10⁻⁸ torr or less in order to preventrecontamination of the atomically clean substrate surface.

An apparatus suited to the process is, then, a mass-selected ion beamdeposition system comprising an ion source, acceleration system for ionbeam transport, mass selection capability for the transmission of onlythe desired species, deceleration system for achieving and controllingthe low ion energies needed for diamondlike film deposition, targetsubstrate assembly, and a deposition chamber maintained at low pressure,preferably with facilities for in situ measurements and controlled gasadmission.

The apparatus used in the development of this invention is described inKang et al., J. Chem. Phys. 88(9), 5882 (1988). This apparatus providedtypical C⁺ current densities of about 500 nanoamps per square centimeterfor a 0.1 square centimeter beam size (for an approximately 100 eVbeam). This corresponds to a deposition rate of only 7 Å per hour(assuming a sticking probability of one and a carbon film density ofthree grams per square centimeter). Such current densities are too lowfor practical thick film depositions but are ideal for in situ filmevolution investigations. The base pressure in the deposition chamber ofthis apparatus is 3×10⁻¹¹ torr while the pressure in the ion source isabout 10⁻⁵ torr. The apparatus was fitted with UHV-compatible leakvalves for the introduction of additional gases (e.g., hydrogen). Thisapparatus is a basic research system not well suited for thick filmproduction due to its low current density. The ion current sharplydecreases when the ion energy is reduced due to space charge repulsionresulting in beam profile dilation. Specially designed beamconfigurations are thus needed for the production of intense ion beamsat energies in the range of 10 to 100 eV.

A major advantage of mass-selected ion beam deposition is the ability tocontrol all the parameters involved in diamondlike film deposition.These advantages are listed below.

(a) Ion species-different types of carbon-bearing ions can be producedby ion sources (e.g., C⁺, C⁻, C_(n) ⁺, hydrocarbon ions, etc.), and, bymass selection, only one ion type be allowed to impinge on the target.

(b) Dual ion beam deposition of carbon ions and hydrogen/argon ions ispossible. Multiple beams can be used to provide for the application ofmore than one species, e.g., dopants may be implanted in a diamondlikefilm as it is being deposited.

(c) The ion energy is controlled in any desired range and the ion energydistribution is relatively narrow.

(d) The ion flux can be controlled in a range covering many orders ofmagnitude (e.g., one nanoamp per square centimeter to one milliamp persquare centimeter).

(e) The ion beam size can be controlled. Rastering of microfocused beamsfor "direct writing" is thus feasible.

(f) Deposition of pure carbon films on atomically clean substratesmaintained under UHV conditions (approximately 10⁻¹⁰ torr) is feasible.Controlled admission of background gases is also possible.

(g) Substrate temperatures can be varied from cryogenic to upwards of1000° C.

(h) In situ substrate cleaning facilities for annealing and sputteringprior to carbon deposition and in situ diagnostics using surfaceanalysis or other techniques are possible.

(i) Post-deposition annealing without exposing the film to theatmosphere is feasible.

Mass-selected ion beam processes can be used to produce films having"tailored" properties. This can be accomplished by varying the specificdeposition parameters which can be supplied by another ion beamdeposition technique or by other deposition methods such as CVD orplasma, depending on the specific deposition parameters required.

Drawbacks to mass-selected ion beam deposition include:

(a) The need for a relatively expensive deposition apparatus whichincludes provisions for ultrahigh vacuum.

(b) Small size of presently available ion beams limits the area that canbe coated on a substrate in a reasonable period of time to only a fewsquare centimeters.

(c) Due to space charge limitations, ion current densities are limited,especially when low deposition energies are involved (less than about100 eV). The maximum deposition rate obtained using the illustratedapparatus is about 3 to 5 Å/sec (1 to 2 micrometers per hour at 100 eVor below). The process has been performed in an apparatus capable ofproviding ion current densities of about 10 mA/cm². Such ion currentdensities are sufficient to deposit a 1-μm thick film in a period ofabout 3 hours.

(d) Only line-of-sight deposition is possible. Thus, complex structurescannot be uniformly coated using this technique. CVD or plasmadeposition techniques are more suitable for such purposes.

The apparatus shown in FIGS. 1 and 2 was utilized to deposit a carbonbased film on a nickel substrate as follows. Briefly, C⁺ ions generatedby electron impact of CO gas were extracted from the ion source regionat the potential of interest, accelerated to high energy, mass selectedto pass ¹² C⁺, and transported through a long flight tube designed toeliminate fast neutrals. The ions were decelerated just prior to impactwith the nickel substrate target. Efficient differential pumping of thebeam line allows maintenance of the system pressure in the low 10⁻¹⁰torr range during exposure to the C⁺ ions. The C⁺ beam currents were inthe range of 10 to 100 nA in a spot size of 0.12 cm² with an energyspread of about 1 eV. The nickel substrate target was a single crystalof Ni with a polished (111) surface which was cleaned by 3 keV Ar⁺sputtering and electron beam annealing to 900° C. before ionbombardment. The techniques of Auger electron spectroscopy (AES), x-rayand UV photoelectron spectroscopy (XPS and UPS), and direct recoil (DR)spectrometry were available in the UHV chamber for sample analysis. Thetotal impurity level (sum of H, C, and O) on the nickel substratesurface after cleaning was <1% of a monolayer as determined by DRspectrometry; AES detected no C or O under these conditions.

In a typical procedure, the ion energy was fixed in the range 10 to 300eV and a clean Ni surface was irradiated with a C⁺ flux that waspredetermined in a Faraday cup mounted on the substrate holder; with ioncurrents of 25 nA, 4 to 5 hours irradiation time was required for a doseof 2×10¹⁶ ions/cm². The beam was interrupted at fixed dose intervals byclosing a gate valve in the beam line and AES spectra were measured inthe derivative mode, i.e., dN/dE, by means of a double-pass cylindricalmirror analyzer using a low current (2 μA) electron beam at 1.6 keV. Thehighly surface-sensitive DR technique was used to monitor the carbon andthe impurity hydrogen and oxygen levels; the impurities were found to benegligible.

The evolution of the C KLL Auger lineshape as a function of 75 eV C⁺dose on Ni(111) is shown in FIG. 5; in order to minimize electron beamdamage processes, a low current (1 microamp) and low energy (1610 eV)electron beam was used for excitation. For initial low C⁺ doses in therange of about 2 to 3×10¹⁵ ions/cm², the lineshape (FIG. 5a) correspondsto that of a carbide (Ni_(n) C). The deposit is approximately onemonolayer thick at this stage. FIG. 5b shows the C lineshapecorresponding to an ion dose of about 6 to 8×10¹⁵ ions/cm². For iondoses of this order, three-dimensional growth of carbon overlayers canbe expected and metal-carbon composite layers must be present. If thisis the case, the C Auger signal must be representative of bothcarbon-carbon and carbon-metal bonds. Indeed, the C lineshape at thisdose stage, called structure (b), is identical to that reported by Craiget al., Surf. Sci. 124, 591 (1983) for sputtered metal-carbon thin filmswith intermediate combinations of amorphous carbon and transitionmetal-carbide lineshapes. This emphasizes the fact that multilayergrowth of the films does not affect the metal-carbon bonding at theinterface. With additional carbon deposition, structures similar tothose shown in FIG. 5c and finally FIG. 5d develop. The AES lineshapeshave been compared with previously report spectra of pure diamond andgraphite samples. The lineshape of FIG. 5c is similar to that of an sp²hybridized "graphitic" carbon form, while that of FIG. 5d is thecharacteristic signature of the sp³ hybridized "diamond" form; both ofthese allotropes nave broad Auger lineshapes. The inset of FIG. 5 showsthe C KLL signature of a deposited carbon film on Ni(111) obtained withminimal exposure to the electron beam; this is precisely the KLLsignature of single crystal diamond. Spectral features deteriorate tothat of FIG. 5d after only a few minutes of exposure to the electronbeam. The results of spectroscopic measurements carried out as afunction of C⁺ energy and dose are discussed at length in Kasi et al.,J. Chem. Phys. 88, 5914 (1988), the teachings of which are incorporatedherein by reference.

The O₂ chemisorption and O⁺ sputtering of the carbon films were testedin situ on structures (a) and (d) of FIG. 5. For chemisorption, thefilms were exposed to 80 Langmuirs of O₂ after which the AES spectrumwas monitored. Oxygen was found to chemisorb readily on (a) with aresulting 50% decrease in the carbon AES signal and simultaneousformation of NiO. No chemisorption was detectable on (d) nor was thereany noticeable carbon removal. The results of low energy O⁺ sputteringshowed that (a) has a sputtering yield of S=1 even below 10 eV, while(d) has a yield of only a few tenths even at O⁺ energies of 20 to 30 eV.Preliminary thermal desorption studies show that the (a) type films arecompletely desorbed below 440° C. while the (d) type films are stable upto temperatures at least 150° C. higher.

These results show that ion beam-deposited carbon films on Ni(111) forman initial reactive carbide layer which evolves continuously withincreasing dose of C⁺ ions into a film with "diamondlike" ASS lineshapewhich is highly resistant to both O₂ chemisorption and O⁺ sputtering. Atthe initial low doses, AES indicates that the surface is covered with adispersed carbide layer in which each carbon atom is bonded directly tothe Ni. With increasing dose, the surface C concentration increases andclustering on neighboring C atoms begins to form a continuous film withC--C bonds. These clusters are chemically bound to the surface throughseveral C--Ni bonds. The fact that films with the latter properties havean energy and dose dependence indicates that the energy of the incomingC⁺ ions is important to the mechanism of film growth and resultingstructure and properties of the film.

Carbide bonded diamond films can be deposited by the method of theinvention on a wide variety of substrate materials. Any solidnonvolatile elemental substance that can be prepared to have anatomically clean surface may be used as a substrate. Especially suitablesubstrate materials are those known to form stable carbide compositions,such as Al, Ba, Ca, Fe, Mn, Mo, Ni, Si, Sr, Ta, Th, Ti, W, V, and Zr.Carbide bonded diamond films have also been successfully applied toelemental substrates which are not otherwise known to form stablecarbides. Hence, by practice of this invention a carbide bonded diamondfilm has been formed on a gold substrate article. Hence, it iscontemplated that Cr, Co, Cu, Pb, Mg, Pd, Pt, Sn and Zn are suitablesubstrate materials for acceptance of a carbide bonded diamond film bydeposit according to the method of this invention. Substrates suitablefor film deposition by the method of this invention may also bemultielement mixture, alloys or compositions.

By employing the method of the invention, carbide bonded diamond filmshave been formed on each of the following substrate materials: nickel,silicon, gold, tantalum, and tungsten. Measurements on the carbon filmsso deposited indicate that they are identical and independent of thesubstrate upon which they are deposited. Carbide bonding to thesubstrate has been unambiguously identified for all five substrates. Thecharacteristics of the deposited films, independent of the specificsubstrate upon which they reside, are discussed immediately below. Amore thorough discussion may be found in Kasi et al., J. Chem. Phys. 88,5914 (1988).

Auger Spectral Characteristics--The Auger electron spectroscopic (AES)results have been presented in FIG. 5 and these results have beendiscussed. Summarizing these results, the first atomic layer depositedhas the AES lineshape of a true carbide compound. The lineshape evolveswith increasing beam dose over the next several atomic layers depositedinto that of true diamond.

Ellipsometry Measurements--Ellipsometry measurement at a wavelength of6300 Å and at several different angles and directions along the crystalprovided a refractive index in the range 2.25 to 2.57; that of purediamond is 2.41. The scatter in the data results from nonuniform filmthicknesses due to the small size of the deposited film relative to theellipsometer photon beam.

X-ray Photoelectron Spectroscopy (XPS)--Results from XPS measurementsusing Mg Kα x-rays are shown in FIG. 6 for a diamond film deposited on aNi(111) substrate. AES had previously been used on this film to showthat it had the diamond lineshape. The XPS spectrum of a freshly cleavedgraphite sample, measured in the same spectrometer system, is also shownin FIG. 6 for comparison to the diamond film. The carbon 1s spectra andcharacteristic energy loss features of graphite and a diamond filmsample are shown in FIG. 6.

The carbon energy loss features of FIG. 6 have been labelled P_(i)according to the spectra for pure diamond and graphite as published byF. R. McFeely, S. P. Kowalczyk, L. Ley, R. G. Cavell, R. A. Pollak, andD. A. Shirley, Phys. Rev. B9, 5268 (1974). The energy position of thevarious features for the published spectra are listed in Table II of theMcFeely et al. publication. The XPS spectra of the diamond filmdeposited by the method of the invention is in agreement with thepublished XPS spectrum of pure diamond and different from graphite inthe following ways:

(a) Graphite, glassy carbon, and microcrystalline graphite exhibit afeature labelled P₁ at 5 to 6 eV above the main C 1s peak labelled P₀.This 5 to 6 eV peak is not observed in the diamond films deposited bythe method of the invention or pure diamond.

(b) The additional energy loss featured in the region labelled P₂ and P₃are distinctly different for the pure diamond and graphite structures asshown in the published spectra by F. R. McFeely et al., Phys. Rev. B9,5268 (1974). The maximum in this feature for the diamond film of thisinvention is at 32 eV and the graphite sample is at 28 eV above P₀. Thereported range for pure diamond is 31 to 34.1 eV and for pure graphiteis 25 to 28 eV.

The origins of these energy loss peaks are as follows. P₀ is the C 1sphotoionization peak. P₁ is due to the plasma oscillation of pπ typeelectrons (π plasmons) for planar sp² bonded carbon atoms. Since diamondinvolves sp³ coordinated carbon, it does not exhibit this feature. Fortetrahedrally coordinated carbon, P₁ is an interband transition whichappears at 11.3 to 12.5 eV above P₀. P₂, P₂ and P₃ arise from plasmaoscillations of valence electrons. Diamond films of this inventionexhibit all of the XPS energy loss features attributed no pure diamondin previous studies.

Ultraviolet Photoelectron Spectroscopic Measurements (UPS)--The UPSspectra measured with He II of (a) clean Ni(111) and the same surfaceafter deposition of (b) 4×10¹⁵ ; (c) 9×10¹⁵ ; and, (d) 3×10¹⁶ C⁺ions/cm² are shown in FIG. 7. The UPS spectrum of graphite is shown inFIG. 5e. The graphite spectrum is in agreement with previous He I:spectra of pure graphite with He II (40.8 eV) radiation reported by J.A. Taylor, G. M. Lancaster, and J. W. Rabalais, J. Amer. Chem. Soc. 100,4441 (1978). The UPS spectrum of the diamond film of the invention isdifferent from that of graphite and identical to the spectrum of purediamond as published by B. B. Pate, Surf. Sci. 165, 83 (1986).

The absence of the peak at 3 eV for the diamond film of the inventionconfirms the absence of pπ type bands. This low energy band extendingfrom the Fermi-level to about 4 eV for graphite is due to the conductionorbitals (pπ type) that are oriented perpendicular to the layers ofgraphite rings. For the diamond film of the invention, a recession ofthe density of occupied states away from the Fermi level is observed,consistent with sp³ hybridization of carbon-carbon bonds and theinsulator characteristics of the diamond structure.

Low Energy Electron Diffraction (LEED)--LEED measurements on the diamondfilms of the invention show that they have an amorphous orpolycrystalline structure.

Valance level Electron Energy Loss Spectra (ELS)--ELS spectra ofgraphite and a diamondlike film deposited on Ni(111) measured withelectron beam energies of 150 and 350 eV are shown in FIG. 8. Theplasmon loss features have been labelled in the figure. P₀ is theelastic scattering peak at the primary energy of 350 and 150 eV. The ELSspectra of the diamond film of the invention are in perfect agreementwith the spectra from pure diamond at both energies as published by P.G. Lurie and J. M. Wilson, Surf. Sci. 65, 476 (1977). Note that in the350 eV spectrum, the large P₁ peak in graphite is totally absent in thediamond film. This peak is due to pπ plasmon loss features at about 7eV, which is characteristic of unsaturated carbon systems having sp²hybridization. The complete absence of this feature in the diamond filmsof the invention is characteristic of sp³ type bonding and a diamondstructure. In the 150 eV spectra, the large P.sub. 1 feature of graphiteis observed at 6.5 eV while the P₁ feature of diamond is much reducedand shifted to 5.0 eV. This P₁ feature in diamond arises from electronicexcitations of discrete levels within the bandgap. The remainingspectral features, labelled P_(n) in FIG. 8, of the diamond film of theinvention are in perfect agreement with the spectra of pure diamond anddistinctly different from those of graphite as published by P. G. Lurieand J. M. Wilson, Surf. Sci. 65, 476 (1977). In summary, the ELS spectraconfirm that the films of the invention are of a diamond structure andnot graphitic.

K-Shell Ionization Loss Spectra (ILS)--ILS spectra for a diamondlikefilm deposited on Si(100) and a pyrolyric graphite sample at a primaryelectron beam energy (E^(p) of 500 eV are shown in FIG. 9. These spectracorrespond to inelastically scattered electrons that have sufferedenergy losses in exciting K-level carbon electrons above the valenceband gap. For graphite, two very intense peaks labelled K₀ and K₁ areobserved. For diamond, K₁ is much more intense than K₀ and thedifference in magnitude reflects the difference in the number of emptystates near the Fermi level. The observation of the K₀ peak in diamondhas been attributed to excitation to empty states in the band gap thatare generated by electron beam damage. The ILS spectra are very similarto natural diamond with the exception of a higher intensity in K₀. Thisnon-negligible intensity at K₀ is due to electron beam damage of thediamond film. The ILS spectra were acquired after the carbon film hadbeen exposed to electron bombardment for purposes of measuring Augerspectra and ELS spectra. In summary, the ILS spectra of the diamondfilms of the invention are in agreement with that of pure diamond aspublished by P. G. Lurie and J. M. Wilson, Surf. Sci. 65, 476 (1977);and S. V. Pepper, Appl. Phys. Lett. 38, 344 (1981).

As noted above, the process may be practiced simultaneously withmultiple mass-selected ion beams. Such multibeam systems allow thegrowth of films having more than one constituent, for example, adeveloping diamond film could be simultaneously doped to produce asemiconductor of high thermal conductivity in a dual beam apparatus.

Preferably the beam lines in a multibeam apparatus are capable ofproducing relatively high current (about 10 to 500 microamps), lowenergy (about 10 to 5000 eV), mass-selected ion beams of useful size(e.g., about 2 cm diameter). The beams may be focused to one spot on thetarget for simultaneous deposition of films composed of two or moreconstituents or for doping of a film with very low concentrations offoreign atoms. Alternatively, multiple beams of the same constituent maybe used to enhance the deposition rate. It should be understood thatthis process is not limited to the growth of pure and doped diamondfilms on substrates, but could be used for growing other films such asSiO₂, SiC, BN, and mixed semiconductors (e.g., GaAs, inSb, etc.).

A dual beam ion deposition system is illustrated schematically in FIG.3. The apparatus consists of a platform on which the following equipmentis positioned: two ion sources (negative ion cesium sputtering ionsources and/or positive gas ion sources), two mass analyzer deflectionmagnets, two einzel lenses, plus equipment to measure and steer the ionbeam and maintain good vacuum conditions. The apparatus is designed toallow ion beam currents from both sources to be produced, transported,directed, and decelerated such that they strike a target spotsimultaneously at energies in the range of about 10 to 5000 eV,depending on the biasing of the target. UHV deposition chamber 14 housesthe sample target, sample manipulator, introduction device, and surfaceanalysis equipment.

In FIG. 3 safety screen 1 (a wire cage) surrounds the beam generatingportion of the apparatus which includes ion source 2, extractor andeinzel lens 3, pump "T" with y deflection plate 4, a 300 liter persecond turbomolecular pump 5 positioned under the pump "T", inflectionmagnet 6, bellows 7, vacuum chamber with 120 liter per second ion pump 8positioned under the chamber, pumping restriction-beam selector aperture9, removable Faraday cup 10, x-y deflection plates 11, einzel focusinglens 12, and deceleration insulator 13. For developmental work,including selection of appropriate parameters, ultrahigh vacuumdeposition chamber 14 is preferably equipped with means for in situsurface analysis techniques.

The cesium sputtering ion source can produce ions of most sputterableelements. It employs cesium ions produced by surface ionization on a hottantalum helix coaxial with a cathode fabricated or coated with theelement to be ionized to induce sputtering from the cathode itself.Beams of H⁻, alkali and transition metals, and nonmetals (C⁻, B⁻, P⁻,As⁻, Si⁻) have been routinely produced. The H beam was produced byloading a titanium metal cathode with H₂ gas and then using the cathodein the ion source. One of these negative ion sources is interchangeablewith an electron impact source for ionization of gaseous molecules andproduction of positive ion beams. When used in this mode, the beam linevoltages are reversed for transport of the positive ions. This sourceallows production of beams of positive atomic and molecular ions (suchas C⁺, CH₃ ⁺, C₂ H₅, etc.) from virtually any compound having sufficientvapor pressure.

In the apparatus illustrated, each ion source is pumped by a 300 literper second turbomolecular pump. Gate valves are used to isolate thesepumps from the sources.

The extractor-einzel lens assembly for each source is preferablyarranged to accelerate the ion beam emerging from the source to amaximum of about 24 keV and to focus the beam through the injectionmagnet onto a pumping restriction-beam selector aperture. The assemblyis preferably of all metal and ceramic construction and bakable to about300° C. The high voltage supplies are most preferably highly regulatedto prevent variation of the total beam energy during deposition.Electrostatic steerers are provided after the extractor-einzel lenses tohelp achieve the correct beam positioning at the pumpingrestriction-beam selector aperture. The deflection magnets have a massenergy product ME/Z² =1.8 at ±30 degrees, wherein E, and Z are the mass(in amu), energy (in MeV) and the charge of the particle, respectively.A mass resolution of at least about 30 should be obtainable.

The large vacuum chamber includes equipment to measure, direct, andfocus the ion beams from the ion sources to the deceleration insulatorand target spot. In the apparatus illustrated, two pumpingrestriction-beam selector apertures are provided at the entrance of thechamber. Two Faraday cups are provided in the chamber to measure beamintensity. The Faraday cups are arranged so that they can be removed orinserted simultaneously to start or stop a dual beam implant orindividually for single beam test purposes. Two x-y electrostaticsteerers in the chamber assist in directing each beam to the same spoton the target. Electrostatic einzel lenses focus each beam as itdiverges from the pumping restriction-beam selector aperture to thetarget. All the components in the chamber are individually mounted withseparate means for adjusting their positions. The chamber is pumped by a120 liter per second sputter-ion pump. The chamber and its componentsare of all metal and ceramic construction and can be vacuum baked to300° C. for an extended period. In the illustrated apparatus thedeceleration insulator is about 2.5 inches long and is equipped withhigh transparent grids at the entrance and the exit to minimize thefocusing effects of the deceleration.

The deposition chamber may be a standard UHV chamber which houses thesample holder that is mounted on a precision manipulator. Ports may beprovided in this chamber for various in situ surface analysisinstruments such as XPS, UPS, AES, LEED, SIMS, TOF ion scattering andrecoiling spectrometry, and inert gas ion sputtering. In the illustratedapparatus, pumping is performed with a 300 liter per second ion pump anda titanium sublimation pump. The entire chamber is bakable to about 300°C. for achieving UHV (less than about 10⁻¹⁰ torr) conditions.

Appropriate beam-producing apparatus including broad-beam (about 2 cmdiameter) high current (about 10 to 500 microamps) low-energy (about 10to 5000 eV) ion sources and power supplies are available from NationalElectrostatic Corporation (Middleton, Wis. 53562). The ion beam linespreferably include acceleration lenses, a magnetic sector for massselection, deceleration lenses, means for tastering, pressure gauges,pumps, power supplies, and the like.

A suitable deposition chamber is available from the Physical ElectronicsDivision of Perkin-Elmer Corporation (Bolder, Colo. 80302). Thisincludes the Model 44UL chamber, Model TNB-X pumping well, Model 04-745linear transport system, and Model 214-0411 titanium sublimation pump.

An appropriate sample introduction and manipulation system is availablefrom the Kurt J. Lesker Company (Clairton, Pa. 15025). This includes theModel VZHPT225 high precision specimen translator, the Model SG-0400MCstainless steel gate valve with copper seal bonnet, and Models VZVPZ38,VZVPZ100 and AZVPZ150 view ports.

A suitable gas introduction system is also available from Kurt J. LeskerCompany. This includes Model MD6 fine control leak valves forintroducing gases into the ion sources and also into the main depositionchamber.

A dual beam system allows the deposition of binary compounds (e.g., BN,SiC, SiO₂) or multilayer compounds. A dual beam deposition system alsoallows for the simultaneous deposition of carbon along with H⁺ or Ar⁺ aswell as the possibility of simultaneous doping.

When attempting deposition on insulators, surface charging byaccumulation of charged particles when no intrinsic source of electronsis available can pose a problem. Plasma discharges always havesufficient electrons available for neutralization. For ion beamdeposition processes surface neutralization can be achieved by providinga stream of electrons from a hot filament.

Investigations utilizing the relatively low current apparatusillustrated in FIGS. 1 and 2 have been directed towards providingrigorous parametric data for diamondlike film growth. This data has beenused to gain a better understanding of the physical and chemicalmechanisms involved in diamondlike film formation and growth and for theestablishment of film growth processes. In these investigations most ofthe unique features of the mass-selected ion beam deposition techniqueare utilized. A brief summary is given below of experimental resultswhich illustrate the system performance. More detailed discussions ofthe experimental results are found in Kang et al., J. Chem. Phys. 88,5882 (1988), Kasi et al., J. Chem. Phys. 88, 5914 (1988), Kasi et al.,Phys. Rev. Lett. 59, 75 (1987), and Rabalais et al., Science 239, 623(1988), the teachings of which are incorporated herein by reference.

The apparatus illustrated in FIGS. 1 and 2 includes means for in situdiagnostics of the chemical nature of the film at different stages ofgrowth. The film evolution at different C⁺ ion doses has beeninvestigated using different surface analysis techniques (AES, XPS, UPS,ELS, ILS, etc.). Several sequential growth stages were detected:carbidic, intermediate, graphitic, and diamondlike. FIG. 5 shows theAuger lineshapes for different stages of growth. Further parametricstudies were based on these results.

Film evolution at room temperature for different C⁺ ion energies in therange of 1 to 300 eV has been investigated. From the data so generated,the ion energy-ion dose phase relationships have been determined. Carbonions in the energy of about 30 to 100 eV were found to be uniformlyefficient for diamondlike film evolution, while at the lower and higherenergies, considerably higher ion doses are required for the evolutionof each stage. The final diamondlike structure has not been obtained forenergies below about 20 eV, emphasizing the important role of ionkinetic energy. A "phase diagram" constructed for the carbidic throughgraphitic transformation is presented in FIG. 4.

The initial monolayer of the deposited film is in the form of a carbidelayer which is chemically bonded to the substrate atoms for substratesnickel (111), silicon (100), polycrystalline tantalum, tungsten, andgold. The film evolves gradually over the next several layers deposited,through intermediate structures, into a diamond structure. The diamondstructure has been confirmed by comparing the results of spectroscopicmeasurements with those of pure diamond and graphite. The phase diagramshown in FIG. 4, prepared as C⁺ ion dose versus C⁺ kinetic energy E,shows the regions of the different structures. The optimum C⁺ energyrange for formation of the diamond structure is about 30 to 175 eV.Below 10 eV the final diamond structure has not been attained and aboveabout 180 eV there is a sharp increase in the dose required to attainthis final structure. Auger depth profiles show that the films are freeof impurities and that the film-substrate interface is sharp. The filmshave been found to be inert to O₂ chemisorption and have a lowsputtering yield.

Film growth on different substrate materials has been investigated. Thesame qualitative behavior has been detected on nickel (111), silicon(100), and polycrystalline gold, which have been rigorously studied. Thesame behavior appears to obtain on surfaces of tantalum, tungsten, andgermanium. Sharp carbidic interfaces that may be most important for filmadhesion have been detected even on substrates for which bulk carbidesare not known. The attenuation behavior of the Auger intensities ofdifferent substrates with C⁺ fluence as illustrated in FIG. 10 indicatesthat the nature of the substrate may affect the mechanism and growthrate for carbon films. Film evolution under a hydrogen backgroundpressure of about 2×10⁻⁷ torr during C⁺ deposition has beeninvestigated. No significant influence of hydrogen pressure on filmevolution was detected. Additionally, co-bombardment with H₂ ⁺ ions atenergies of about 0.5 to 1.5 keV caused no detectable changes in thefilm evolution sequence.

Inasmuch as hydrogen is used in many CVD processes for the deposition ofdiamondlike films, an experiment was performed to determine the effectof the presence of hydrogen during a deposition performed by the processof the present invention. Since hydrogen was not incorporated into thedeposited film, it is contemplated that the low energy C⁺ ions displacechemisorbed hydrogen from the surface. This is not the case for mostresidual gases and thus the deposition chamber must be substantiallyfree of species such as water vapor, carbon monoxide, carbon dioxide,oxygen, and nitrogen if essentially pure carbon (or other singlespecies) films are desired. In general, the combined partial pressuresof residual gases containing carbon, oxygen, and/or nitrogen in thedeposition chamber should be less than about 10⁻⁸ torr, more preferably,less than about 10⁻⁹ torr.

Post-deposition annealing of diamondlike films on Ni(111) and gold hasalso been carried out. The diamondlike films on nickel substrates werefound to be unstable under annealing. AES analysis could detect nosubstrate signal on the as-deposited film prior to annealing. At 200°C., nickel Auger lines appeared. It is contemplated that this maypossibly be due to some Ni-C interdiffusion or film recrystallizationand island formation. Carbon dissolution in the nickel was detected byan increase in the nickel LMM intensity and a decrease in the carbon KLLline intensity at temperatures exceeding 400° C. A graphitictransformation observed by the changes of the C Auger lineshape wasassociated with this dissolution. No carbon dissolution was observed onthe gold substrates, even at temperatures of 875° C. A pronouncedgraphitic transformation occurred only at temperatures higher than 600°C.

Film resistance to attack by thermal O₂ and to sputtering by differentionic species (O⁺, Ne⁺, Ar⁺) at different energies (about 10 to 300 eV)has been investigated. The diamondlike films obtained by the process ofthe present invention were found to be inert to O₂ chemisorption, unlikegraphite or carbide films. The sputtering yield by the ionic species wasat least three times lower for the diamondlike film than for carbidefilms or for the native substrate material.

The nature of the carbon deposit and the evolution of the film can beintimately followed by means of the carbon Auger lineshape. Theselineshapes serve as a fingerprint of the chemical environment. Theevolution of the carbon KLL Auger lineshape as a function of C⁺ dose onNi(111) is shown in FIG. 5. For an initial C⁺ dose in the range ofapproximately 2×10¹⁵ to 3×10¹⁵ ions per square centimeter, the lineshape(FIG. 5a) corresponds to that of a carbide (Ni_(n) C). The deposit isapproximately one monolayer thick at this stage. FIG. 5b shows thelineshape corresponding to a dose of approximately 6×10¹⁵ to 8×10¹⁵ ionsper square centimeter, for which metal-carbon composite layers andthree-dimensional carbon overlayers begin to grow. With additional C⁺deposition, structures characterized by the spectra in FIGS. 5c and 5ddevelop. The lineshape of FIG. 5c is similar to that of sp² hybridizedgraphite, whereas that of FIG. 5d is the characteristic signature of sp³hybridized diamond. The distinction between diamond and graphiteallotropes is even more pronounced in the low-energy portion of thespectrum. Graphite spectra exhibit two peaks at approximately 80 andapproximately 110 eV that are absent in diamond spectra.

The characteristic carbon Auger lineshapes have been used to map out acarbon phase diagram for C⁺ deposition on Ni(111) as a function of C⁺ion dose and energy. This phase diagram is shown in FIG. 4. The AESlineshape (FIG. 5d) evolves smoothly from that of FIG. 5c and it is notpossible to assign an exact dose for the crossover point. Hence a phasecorresponding to that of FIG. 5d has not been indicated in FIG. 4. Forion energies below 10 eV, the carbon lineshapes do not evolve completelyinto the final structure of FIG. 5d. In the range from 30 to 175 eV, thetransformations are nearly energy-independent. This is the optimumenergy for diamond film deposition. For energies greater than about 180eV, there is a sharp increase in the dose necessary to attain thedifferent phases. In this region, the increasing significance ofself-sputtering, lattice damage, and penetration by the C⁺ ions isthought to be responsible for this phenomenon. For carbon deposition onSi(100) and gold surfaces, the doses necessary to achieve the sametransitions as shown in FIG. 5 were higher than those on Ni(111). Thissensitivity to this type of substrate surface is thought to arise fromthe intimate carbide registry with the surface.

Low energy ion beams can be used to deposit films of materials inmetastable states. The energy deposited by the ion beam results in alocalized transient "thermal spike." It is contemplated that this highenergy content region is rapidly quenched by dissipation of energy intothe solid. The result is that the deposited material can be trapped inan excited metastable state that persists after the thermal spike hasbeen quenched. For example, using the process of the present invention,carbon ion beams have been used to deposit films of carbon in such ametastable state (i.e., diamond). It is contemplated that similarmetastable materials can be prepared from silicon, boron, phosphorous,sulfur, and germanium.

Ion beams are ideally suited to deposition of diamond films inside smallholes, apertures, passages, and the like. The ion beam can be directedinto apertures that have very small diameters but are long. For example,the process of the present invention has been used to deposit a diamondfilm on the inside surface of a hole 0.030 inch in diameter and 1 inchlong. Some of the carbon ions pas through the hole and emerge at theopposite end; however, there is sufficient divergence in the beam suchthat ions strike the walls of the aperture and coat it with adiamondlike film.

Thin diamond films can be used for rapid cooling in cryogenicapplications. For example, freezedrying of tissues is typicallyaccomplished by rapidly pressing a tissue sample onto a copper blockcooled to liquid nitrogen or liquid helium temperatures. There are atleast two problems with this. First, the copper block has an oxidecoating which is a poor conductor. Second, heat dissipation is limitedby the thermal conductivity of the oxide and copper metal. Both of theselimitations can be overcome by drilling very tiny holes into the copperblock and coating the block and interior of the holes with a diamondfilm. The film prevents formation of the oxide layer and has a higherthermal conductivity than copper. It also facilitates heat dissipationboth laterally along the surface as well as perpendicularly into thebulk of the copper.

As noted above, diamond films can be doped to make semiconductors. Thisis preferably accomplished by using two beams which impinge on asubstrate simultaneously. One is used to deposit the diamond while theother supplies the dopant. The level of doping may be controlled byadjusting the ion current in the beams. Low energies are preferably usedso as not to destroy the sp³ hybrid bonding in the diamond structure. Analternative method is to place a container of the dopant near thesubstrate and heat the container so that the dopant vapor deposits onthe substrate simultaneously with the carbon ions from the ion beam.

Semiconducting diamond is an attractive material for integrated circuitsbecause of its large bandgap, high carrier mobilities and saturationvelocities, high breakdown field, radiation hardness, and unsurpassedthermal conductivity. These properties suggest the possibility ofultrahigh speed integrated circuits, capable of operating attemperatures upwards of 1000° C. Perhaps the most direct application isin the area of millimeter wave power amplifiers and oscillators.However, other critical potential applications are high speed dataprocessing circuits, supercomputers, and high temperature sensing.

Diamond is known to have a large band gap; those of silicon andgermanium are much smaller. Thus, a diamond-based semiconductor offers abetter opportunity to tailor the position of the energy levels by theintroduction of selected dopants. This implies that improved performanceat high frequencies may be had with such a semiconductor. The mobilityof charge carriers in diamond is very high and, like semiconductors andunlike most metals, the mobility increases with increasing temperature.

Dopant ionization energies in diamond are typically near 0.3 eV. Thus,only a small fraction of dopant is ionized at room temperature. As aresult, diamond devices operating at room temperature exhibit highseries resistances due to both the resistance of the bulk semiconductorand of the tunneling contacts. Efficient dopant ionization requirestemperatures above about 500° C. This high temperature operatingrequirement must be considered at the beginning of any diamond deviceprogram. It would seem that many technologies which have been developedfor silicon (e.g., PSG, aluminum metallization, etc.) will beinappropriate for diamond. Advances in packaging, interlevel dielectric,metallization, and contact technologies must be made beforediamond-based integrated circuits can be practical. The necessity forhigh temperature operation implies that the ultimate speed of diamonddevices is still in question. Indeed, the high mobility values (e.g.,2000 cm² /V-sec) often quoted are obtained at room temperature. Hightemperature values should be smaller. However, it has been shown thatthe breakdown field at high temperature is good and high temperatureoperation of a permeable base transistor has been demonstrated.

Doping of diamond is a critical problem. Ion implantation results indefects which cannot be annealed. Under certain conditions these defectsact as donors, a result which has been used to fabricate transistors inp-type natural diamond. Doping during microwave plasma CVD growth hasbeen demonstrated for both arsenic and boron. Schottky contacts havebeen made from a variety of materials including gold, nickel andtungsten. The most important of these are clearly nickel and tungstenbecause of the high thermal stability of the metal itself and also ofthe carbide which probably forms at the diamond-tungsten interface. Ithas been shown that low pressure chemical vapor deposition (LPCVD)silicon dioxide can produce very low surface state densities. Thisresult is critical to development of diamond MOSFETs.

Diamond films on nickel have been found to be unsuitable for hightemperature application. Silicon is preferred on technological grounds.This is principally because an extensive crystal growth and fabricationtechnology already exists and also because silicon's protective oxidemakes high temperature operation more reliable than nickel. Siliconsubstrates have a rather poor lattice match with diamond; the latticeshave a common period of three diamond lattice constants to two siliconlattice constants. This implies the potential for epitaxial growth sincethere is no strain in the bulk epitaxial material even though theinterface has considerable defects. Preliminary x-ray diffraction dataof diamond films deposited by the process of the present invention on aSi(111) surface shows that the diamond lattice is epitaxially orientedto the silicon lattice. It is contemplated that the non-planar devicestructures used for GaAs epitaxial transistors and MOSFETs will be mosteasily transferred to diamond film technology. It if furthercontemplated that a permeable base transistor could be constructed usingall-epitaxial technology. Such a device can be made by existingdirect-write electron beam lithography processes with a provenresolution of 0.025 microns and an alignment accuracy of 0.05 microns.Systems for metallization, LPCVD oxide and nitride, LPCVD tungsten, ionassisted evaporation, sputtering, and reactive ion etching areavailable.

The low energy, room temperature, mass analyzed ion beam depositionmethodology of the present invention has been shown to producechemically bonded films on a variety of substrates. Such films may becrystalline, polycrystalline, or amorphous. The method comprises a dosedependent deposition procedure to assemble stoichiometric films.However, it can also be used as a deposition procedure to assemblenon-stoichiometric films. Such films may exhibit high degrees ofreactivity because of intrinsic unsaturated valences and therefore canbe excellent catalysts for a variety of reactions.

The deposition of diamond films on a variety of substrates would makepossible the production-of many articles heretofore unattainable.Sharper and more durable machine tools and other cutting tools likeknives and surgical scalpels could be made by applying a diamond film tothe cutting surfaces of such articles. Magnetic data storage media,aircraft windows, and optical lenses and the like, if coated with adiamond film, would be more abrasion resistant. Coated with a diamondfilm, silicon or other substrate materials such as nickel could beemployed for the production of high density integrated circuits and highpower semiconductors. For example, transistors could be fabricated fromdiamond by adding dopants to give the diamond semiconductor properties.Such transistors could handle high power signals at microwavefrequencies.

The deposition technique of the present invention can be used to producecompound thin films from elements that do not typically react. Thedeposition process can be used to form metastable phases of carbon,silicon, phosphorous, germanium, tin, sulfur, selenium, and gallium.

This process may also be used for the deposition of cubic (diamondlike)boron nitride as well as layered boron nitride (graphitelike). The cubicform of boron nitride is known to scratch diamond itself. The activationenergy for transformation to the cubic form is derived from the ion beamenergy. Nitridation with low energy (approximately 1 to 150 eV) N⁺ ionsof boron is the deposition route.

The process is also applicable to the deposition of thin films ofborides such as ZrB₂, AsB₆ and TiB₂ by using mass-selected boron ions.

The deposition process may be used to produce thin films of carbides ofvarious metals. These include, but are not limited to, silicon, boron,germanium, copper, silver, gold, zinc, cadmium, beryllium, aluminum,yttrium ytterbium, cerium, nickel, tungsten, tantalum, titanium, andcalcium.

The process of the present invention may be used to form "zeolite-like"material films from the multistep deposition of individual components.For example, bombardment with aluminum and silicon successively of ametal oxide may lead to oxygen segregation and random metal oxidecrystallites may be formed on the surface.

This method may be used to create thin films with propertiesintermediate between those of the known allotropic forms.

The process may be used to deposit thin films on a material thatexhibits no reactivity towards the substrate at thermal energies, suchas a carbide film on germanium. The ion beam translational energy isused to overcome reaction barriers.

The deposition method of the present invention can be used in situationswherein films of a particular species to be deposited are chemicallybonded to substrates to which they do not typically react. The idea isto form a monolayer of atoms other than the film material itself at thefilm-substrate interface. This species should be chosen for its abilityto chemically bind the substrate surface atoms to the underlayer of thefilm material itself.

This deposition method combines the chemical reactivity of the impingingspecies with physical momentum transfer. The consequent elimination ofall other species foreign to the film composition aids in theminimization of detrimental surface modifications related to, forexample, plasma exposure.

Preparation of coating surfaces is necessary to ensure good adhesion ofthe film and prevent occlusion of impurities. Cleanliness on an atomicscale is possible by use of a low dosing, reactive ion beam operating atlow energy. Most preferably, the source beam is composed of ions of thedeposition material itself.

The process may be used to produce a free-standing diamond film bydepositing the film on a substrate which is subsequently dissolved orotherwise removed.

Although ultrahigh vacuum conditions are preferred, at high flux, thebeam itself may keep the surface atomically clean even at higherpressures.

The process of the present invention can be used to grow an epitaxialdiamond layer on monocrystalline silicon with the crystallographic axesaligned with that of the silicon.

Although ion beam deposition is most directly applicable to depositionson surfaces positioned to be substantially perpendicular to the focusedion beam, the inherent divergance in such beams can be employed to coatthe surfaces of articles not directly accessible to a line-of-sightdeposition. If a diverging ion beam is directed into-the vicinity of asurface ions from the beam can nonetheless impact onto the surfacethereby providing the desired thin coating. The decelerator may bespecifically tailored to provide the requisite-degree of divergance inthe ion beam.

The foregoing description has been directed to particular embodiments ofthe invention in accordance with the requirements of the United Statespatent statutes for the purposes of illustration and explanation. Itwill be apparent to those skilled in this art, however, that manymodifications and changes in the methods and compositions set forth willbe possible without departing from the scope and the spirit of theinvention. It is intended that the following claims be interpreted toembrace all such modifications and changes.

We claim:
 1. A method for depositing a chemically bonded carbon basedfilm having a diamond structure on a substrate, comprising the stepsof:positioning an electrically grounded substrate within a depositionchamber; preparing the substrate to have an atomically clean surface andmaintaining said surface in an atomically clean state during depositionby maintaining the pressure within the deposition chamber at or below1×10⁻⁸ torr; generating from a carbon containing material a source ofgaseous ions in an ionization region maintained at 1 to 300 eV above thegrounded substrate; directing ions from the ionization region intoelectromagnetic mass analyzer; selecting ¹² C⁺ ions to exit from themass analyzer; directing said ¹² C⁺ ion beam through a bend by passingsaid beam through a parallel plate condenser to eliminate line-of-sightneutral particles from said ion beam; directing said ¹² C⁺ ion beamthrough a decelerator lens immediately in front of said substrate;impinging said ion beam on the atomically clean substrate surface at abeam energy sufficient to form a carbon layer bound to said substratesurface by carbide bonding; and, impinging said ion beam on said carbidebound carbon layer at a beam energy of at least about 10 eV for a timesufficient to overlay said carbide bound carbon layer with a diamondlayer, said diamond layer being substantially free from hydrogen oroxygen.
 2. The method of claim 1 wherein the ion beam is impinged uponsaid substrate at an energy of from about 10 eV to about 250 eV.
 3. Themethod of claim 1 wherein the energy of the impinging ion beam is fromabout 30 eV to about 175 eV.
 4. The method of claim 1 wherein thesubstrate is Al, Ba, Ca, Fe, Mn, Mo, Ni, Si, Sr, Ta, Th, Ti, W, V, Zr,an alloy or mixture thereof.
 5. The method of claim 1 wherein thesubstrate is Cr, Co, Cu, Pb, Mg, Pd, Pt, Sn, Zn, an alloy or mixturethereof.
 6. The method of claim 1 wherein the substrate is Ni, Si, Au,Ta, or W.
 7. The process recited in claim 1 wherein the substratecomprises magnetic data storage media.
 8. A carbon based film depositedby ion beam deposition upon a substrate, comprising:an anterior carbonlayer substantially free of hydrogen or oxygen and having a diamondmicrostructure wherein the anterior layer overlays and is chemicallybonded to a posterior atomic layer of carbon that is chemically bound tosaid substrate surface by carbide bonding.